Performing a coordinate rotation digital computer (CORDIC) operation for amplitude modulation (AM) demodulation

ABSTRACT

In one aspect, the present invention includes a method for receiving an amplitude modulation (AM) signal in a receiver and performing a coordinate rotation digital computer (CORDIC) operation in obtaining a demodulated AM signal. The demodulated AM signal may be obtained from a magnitude output of the CORDIC operation or as a real output of a multiplication between a complex baseband signal and a demodulating carrier signal generated in a feedback loop.

Embodiments of the present invention relate to radio receivers, and moreparticularly to such receivers including an amplitude modulation (AM)receiver.

BACKGROUND

Radio receivers such as AM and frequency modulation (FM) receivers arewell known and are pervasive. Conventionally, these receivers have beenformed of analog circuitry to receive an incoming radio frequency (RF)signal, downconvert the signal, and demodulate the downconverted signalto obtain an audio signal for output. Typically, the circuitry for AMand FM receivers, even in a combined radio, includes separate dedicatedpaths for AM and FM operation. While such analog-based circuitry mayperform well, the area associated with this analog circuitry typicallyexceeds that used for digital circuitry, and the analog receiverstypically include many discrete components. In contrast, digitalcircuitry is generally available in ever-decreasing sizes, as thebenefits of advanced semiconductor processes provide for greaterintegration benefits. Furthermore, the cost of digital integratedcircuits (ICs) is generally less than corresponding analog circuitry.

Accordingly, some radio receivers are being designed to incorporategreater amounts of digital circuitry. While such circuitry may improveperformance and can be formed in small packages, typically there arecomplexities in processing RF signals that require significant digitalprocessing to match the relatively simple circuitry of an analogreceiver.

Additional issues exist in radio receivers. One such issue is finelytuning to a desired frequency. To effect such fine tuning, manyreceivers include an automatic frequency control (AFC) circuit toreceive a feedback signal from a downconverted incoming signal in aneffort to finely tune a local oscillator (LO) that is used to downmixthe incoming signal to the desired frequency. Typically such AFCcircuitry is present in a front end of the receiver to receive an outputof the downconverting mixer and, via the action of feedback, attempts tofinely tune the LO to obtain the desired channel frequency. However,this circuitry raises complexity and consumes additional circuit areaand power.

SUMMARY OF THE INVENTION

In one aspect, the present invention includes a method for receiving anamplitude modulation (AM) signal in a receiver and performing acoordinate rotation digital computer (CORDIC) operation in obtaining ademodulated AM signal. The demodulated AM signal may be obtained from amagnitude output of the CORDIC operation or as a real output of amultiplication between a complex baseband signal and a demodulatingcarrier signal generated in a feedback loop. A phase output of theCORDIC operation may be provided to the feedback loop, which maygenerate different values to provide for the multiplication, dependingon a selected output for the demodulated signal.

Another aspect of the present invention is directed to an apparatus thatincludes a receiver to receive an AM signal and generate a complexbaseband signal therefrom, a combiner to receive the complex basebandsignal and a demodulating carrier signal of a feedback loop and togenerate a complex output therefrom, and a CORDIC engine to perform aCORDIC function on the complex output to obtain a polar result. Incertain implementations, a magnitude of the polar result may correspondto a demodulated signal, while in other implementations a real portionof the combiner output may correspond to the demodulated signal. In someembodiments, a controller may select the combiner output or the CORDICengine output as a source of the demodulated signal. The CORDIC enginemay be a software routine performed by a processor such as a digitalsignal processor (DSP).

In one implementation, the feedback loop may be coupled to an output ofthe CORDIC engine to generate the demodulating carrier signal, and mayinclude a loop filter coupled to an output of the CORDIC engine and acontrolled oscillator coupled to an output of the loop filter, where thecontrolled oscillator generates the demodulating carrier signal. Thefeedback loop may be used to enable automatic frequency control of theAM signal without a dedicated detector. In one embodiment, the feedbackloop may include a loop filter coupled to an output of the CORDIC engineand a controlled oscillator coupled to an output of the loop filter.

In one implementation, the method may be performed using a combinedAM/FM receiver in which operation may be controlled to be in AM or FMmode. In such an implementation, software for the AM mode of operationmay be executed in a DSP responsive to selection of the AM mode, whilesoftware for the FM mode may be executed in the DSP responsive toselection of the FM mode.

While embodiments may be implemented in many different forms, in oneembodiment an apparatus may take the form of an integrated circuit (IC)including a digital demodulator and a feedback loop as described above.Furthermore, some implementations may be incorporated in a receiver thatincludes a mixer to downconvert an incoming signal to an intermediatefrequency (IF) signal, a digitizer to digitize the IF signal, afrequency synthesizer to downconvert the digitized IF signal to abaseband signal, a demodulator to demodulate the baseband signal togenerate a demodulated signal using a CORDIC operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multimode transceiver in accordance withone embodiment of the present invention.

FIG. 2 is a block diagram of an AM receiver in accordance with anembodiment of the present invention.

FIG. 3 is a block diagram of an AM receiver in accordance with anotherembodiment of the present invention.

FIG. 4 is a block diagram of a controlled oscillator in accordance withone embodiment of the present invention.

FIG. 5 is a block diagram of an automatic volume control circuit inaccordance with one embodiment of the present invention.

FIG. 6 is a block diagram of a system in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

In various embodiments, an incoming radio frequency (RF) signal can bedownconverted to a complex baseband signal that can then be processed byvarious circuitry to obtain a demodulated signal. Computations then maybe performed on the complex signal to obtain magnitude information thatmay correspond to a demodulated signal. At the same time, phaseinformation generated from the complex signal may be used in a feedbackloop to perform automatic frequency control (AFC). Furthermore, in someembodiments automatic volume control (AVC) may be efficientlyimplemented on the demodulated signal to obtain an output that maintainsa substantially steady volume, even in the presence of channelinterference, fading or the like.

While the scope of the present invention is not limited in this regard,in some embodiments an AM receiver may be implemented using asubstantial amount of digital circuitry, and an entire AM receiver maybe implemented on a single integrated circuit (IC) having both analogfront-end circuitry to receive and downconvert an incoming RF signal, aswell as digital circuitry such as a digital signal processor (DSP) orother digital circuitry to process the baseband signal to obtain ademodulated output. Furthermore, in some implementations an AM receivermay be combined in a single IC (e.g., on the same monolithic die) withan FM receiver/transmitter (i.e., transceiver). In such embodiments, asubstantial amount of circuitry for both AM and FM reception modes, aswell as for FM transmission, may be reused. In some implementations, toeffect such operations a single set of receiver circuitry may beprovided, and a different firmware image may be selected for operationin an AM or FM mode. Owing to the relatively small size of such an IC,embodiments may be configured for use in portable devices, such ascellular telephones, personal media players such as MP3 players andpersonal digital assistants (PDAs), although the scope of the presentinvention is not limited in this regard.

Referring now to FIG. 1, in accordance with embodiments of the inventiondescribed herein, a multimode combined AM/frequency modulation (FM)transceiver 10, which may be fabricated on a monolithic semiconductordie 11, has several different signal processing modes of operations, inwhich the transceiver 10 may perform FM transmission, AM or FMreception, analog mixing, digital mixing and codec functions. Morespecifically, as described herein, the multimode FM transceiver 10 hasan FM transmit mode in which the transceiver 10 functions as an FMtransmitter; an AM or FM receive mode in which the transceiver 10functions as a receiver; and an audio mode in which the transceiver 10functions as a codec. In each of these modes of operation, the multimodetransceiver 10 may perform various analog and/or digital mixingfunctions. Additionally, in accordance with some embodiments of theinvention, the multimode transceiver 10 includes a digital audiointerface 16, which allows the communication of digital audio signalsbetween the transceiver 10 and circuitry (“off-chip” circuitry, forexample) that is external to the transceiver 10.

In accordance with embodiments of the invention the FM transmit, AM andFM receive and audio modes are orthogonal in that the multimodetransceiver 10 is in only one of the modes at a time. However, it isunderstood that in other embodiments of the invention, the multimodetransceiver may operate in two or more of the modes concurrently. Ingeneral, the multimode transceiver 10 may receive one or more of thefollowing input source signals in accordance with some embodiments ofthe invention: a digital audio (called “DIN”), which is received throughthe digital audio interface 16; an incoming RF signal that is receivedfrom an external receive antenna 80; a digital audio band signal that isreceived from the digital audio interface 16; and left channel (called“LIN”) and right channel (called “RIN”) analog stereo channel signalsthat are received at input terminals 40 and 42, respectively.

Depending on the particular configuration of the multimode transceiver10, the transceiver 10 is capable of mixing two or more of its inputsource signals together to generate one or more of the following outputsignals: an outgoing FM transmission signal to drive an externaltransmit antenna 60; left channel (called “LOUT”) and right channel(called “ROUT”) analog stereo signals that appear at output terminals 52and 50, respectively; and a digital output signal (called “DOUT”) thatis routed through the digital audio interface 16. In accordance withsome embodiments of the invention, the multimode transceiver 10 may alsoprovide a low impedance RF transmission output signal (called “TXB”) atan output terminal 64 for purposes of driving a low impedance load.

As described herein, the multimode transceiver 10 may reuse some of itshardware components for purposes of reducing the complexity and size ofthe transceiver 10, as well as reducing the overall time that may beconsumed designing the transceiver 10. For example, in accordance withsome embodiments of the invention, a digital signal processor (DSP) 20of the multimode transceiver 10 performs both digital FM modulation (forthe FM transmit mode) and digital AM and FM demodulation (for thereceive mode) for the transceiver 10. As another example of the hardwarereuse, analog-to-digital converters (ADCs) 24 and 26 of the multimodetransceiver 10 perform transformations between the analog and digitaldomains for both complex (when the transceiver 10 is in the FM receivemode) and real (when the transceiver 10 is in the transmit modes)signals. Additionally, the ADCs 24 and 26 may be used in the audio modefor purposes of digitizing the LIN and RIN stereo channel signals.

As another example of hardware reuse by the multimode transceiver 10, inaccordance with some embodiments of the invention, digital-to-analogconverters (DACs) 32 and 36 of the transceiver 10 convert digital audioband signals from the digital to the analog domain for both the receiveand audio modes. The DACs 32 and 36 are also used during the FM transmitmode for purposes of converting intermediate frequency (IF) band signalsfrom the digital to the analog domain.

Turning now to the overall topology of the multimode transceiver 10, thetransceiver 10 includes a multiplexer 95 for purposes of routing theappropriate analog signals to the ADCs 24 and 26 for conversion. Forexample, the multiplexer 95 may select an incoming analog IF signalduring the receive mode and select the LIN and RIN stereo channelsignals during the FM transmit and audio modes. The digital signals thatare provided by the ADCs 24 and 26 are routed to the DSP 20.

For the receive modes, the multimode transceiver 10 includes analogmixers 90 that are coupled to a tunable local oscillator 92, thefrequency of which selects the desired radio channel to which thetransceiver 10 is tuned. In response to the incoming RF signal, themixers 90 produce corresponding analog IF, quadrature signals that passthrough programmable gain amplifiers (PGAs) 94 before being routed tothe ADCs 24 and 26. Thus, the ADCs 24 and 26 convert the analog IFquadrature signals from the PGAs 94 into digital signals, which areprovided to the DSP 20. The DSP 20 demodulates the received complexsignal to provide corresponding digital left and right channel stereosignals at its output terminals; and these digital stereo signals areconverted into the analog counterparts by the DACs 32 and 36,respectively. As described further below, mixing may then be performedby mixers, or analog adders 54, which provide the ROUT and LOUT stereosignals at the output terminals 50 and 52, respectively. It is notedthat the digital demodulated stereo signals may also be routed from theDSP 20 to the digital audio interface 16 to produce the DOUT digitalsignal.

In the FM transmit mode of the multimode transceiver 10, the content tobe transmitted over the FM channel (selected by the frequency of thelocal oscillator 92, for example) may originate with the DIN digitaldata signal, the LIN and RIN stereo channel signals or a combination ofthese signals. Thus, depending on whether the analog signals communicatesome or all of the transmitted content, the multimode transceiver 10 mayuse the ADCs 24 and 26. The DSP 20 performs FM modulation on the contentto be transmitted over the FM channel to produce digital orthogonal FMsignals, which are provided to the DACs 32 and 36 to producecorresponding analog orthogonal FM signals, which are in the IF range.Analog mixers 68 (which mix the analog orthogonal FM signals with afrequency that is selected by the local oscillator 92) of the multimodetransceiver 10 frequency translate and combine the signals to produce anRF FM signal that is provided to the transmit antenna 60. In the audiomode of the multimode transceiver 10, the DSP 20 may be used to performdigital mixing. Analog mixing in the audio mode may be performed usingthe adder 54.

Among the other features of the multimode transceiver 10, in accordancewith some embodiments of the invention, the transceiver 10 includes acontrol interface 38 for purposes of receiving various signals 39 thatcontrol the mode (FM transmit, AM or FM receive or audio) in which thetransceiver 10 is operating, as well as the specific submodeconfiguration for the mode, as further described below. For example,different firmware present in the DSP 20 may be executed based on theselected mode of operation. In accordance with some embodiments of theinvention, the multimode FM transceiver 10 may also include amicrocontroller unit (MCU) 98 that coordinates the general operations ofthe transceiver 10, such as configuring the ADCs 24 and 26 and DACs 32and 36, configuring data flow through the multiplexer 95, or the like.

Using the transceiver 10 of FIG. 1 (for example), digital demodulationof incoming RF signals may be performed. In AM operation, a received AMsignal, r(t), has the form:r(t)=k(A+m(t))cos(ω_(o) t+Θ)  [1]

In Equation 1, the message is m(t). In turn, ω_(o) is the carrier signaland Θ corresponds to a phase of the transmitter, which is unknown at areceiver, and k is a value associated with a communication channel. ForAM, A is chosen so that A+m(t) is always greater than zero. Manytransmitters are configured such that A is set to 1 to guarantee thatA+m(t) is greater than zero. Note that m(t) can vary with the message,and k can vary with the environment in which the signal is transmitted.

For AM demodulation, the amplitude of the signal may be determined. Sucha determination may be aided in a receiver that performs a complex firststage demodulation. In such a system, the incoming signal is firstdemodulated to an IF signal. Both in-phase (I) and quadrature (Q)components may be demodulated to an IF signal. Hence, there are twocomponents to the AM signal:r _(I)(t)=k(A+m(t))cos(ω_(IF) t+Θ)  [2]r _(Q)(t)=k(A+m(t))sin(ω_(IF) t+Θ)  [3]In Equations 2 and 3, ω_(IF) may correspond to the frequency of thecarrier signal plus or minus the local oscillator frequency used todownconvert the RF signal to IF. To eliminate the carrier signal, themagnitude may be computed as follows:R(t)=(r _(I) ²(t)+r ² _(Q)(t))^(1/2)=(k ²(A+m(t))²[cos²(ω_(IF)t+Θ)+sin²(ω_(IF) t+Θ))]^(1/2) =k(A+m(t))  [4]

In various embodiments, a coordinate rotation digital computer (CORDIC)function such as may be performed via a CORDIC coprocessor or bysoftware executing on generic hardware can be used to obtain themagnitude information. Note that in these embodiments, the squaring,summing, and square root operations described above may be implementedas a single operation, since a CORDIC engine may convert rectangularcoordinates (i.e., x, y) to polar coordinates (r, Θ), and therelationship between x, y and r is:r=(x ² +y ²)^(1/2)  [5]

Referring now to FIG. 2, shown is a block diagram of an AM receiver inaccordance with an embodiment of the present invention. In someembodiments, receiver 100 of FIG. 2 may be implemented via the variouscomponents of transceiver 10 described above, although the scope of thepresent invention is not limited in this aspect. As shown in FIG. 2,receiver 100 is used to receive and process an incoming AM signal.Receiver 100 includes an antenna 105 to receive an RF signal and provideit to a low noise amplifier (LNA) 110. The output of LNA 110 is providedto a complex mixer 120 which generates I and Q signals therefrom. Whilenot shown in the embodiment of FIG. 2, in various implementations mixer120 may be controlled by an output of a voltage controlled oscillator(VCO) or a numerically controlled oscillator (NCO). The complex outputsof mixer 120 may be amplified in programmable gain amplifiers (PGAs) 125a and 125 b. PGAs 125 a and 125 b may operate based on automatic gaincontrol (AGC) to output a signal of substantially steady gain, in someembodiments. Complex mixer 125 may mix the incoming RF signals down to alow intermediate frequency (IF) value. In various instances, incomingsignals may be provided with positive gain (i.e., amplification) ornegative gain (i.e., attenuation) in PGAs 125, based on variouscircumstances. The output of PGAs 125 a and 125 b may be provided tocorresponding analog-to-digital converters (ADCs) 130 a and 130 b. Theoutputs of ADCs 130 a and 130 b may be provided to a direct digitalfrequency synthesizer (DDFS) 135 that may generate a downmixed basebandcomplex signal, which may in turn be filtered via a low pass filter(LPF) 140. The filtered complex signal may then be multiplied by acomplex exponential (described further below) in a combiner such as amultiplier 145. The output of multiplier 145 may be provided to a CORDICengine 150 that performs CORDIC operations on the incoming value toobtain both magnitude and phase information, i.e., polar coordinateinformation.

In one embodiment, CORDIC engine 150 may be implemented using minimalamounts of hardware, e.g., adders, accumulators and comparators, whichmay be operated according to a state machine. Furthermore, a smalllookup table that includes a minimal amount of values for differentincoming information may also be present. The magnitude portion of theCORDIC function may correspond to a demodulated signal. Thus as shown inFIG. 2, output 152 of CORDIC engine 150 may correspond to thedemodulated AM signal, which may be provided to one or more desiredlocations such as an output device, e.g., a speaker, a storage deviceand so forth.

Referring still to FIG. 2, note that the phase portion of the output ofCORDIC engine 150, i.e., output 154, may be provided to a differentiator155. The output of differentiator 155, which may be a varying voltage inresponse to a constant input, or a constant voltage in response to avarying input, may be provided to a loop filter 160 which may be an LPFthat in turn is coupled to an NCO 165, which generates the complexexponential, which corresponds to a demodulating carrier signal. Thisfeedback loop acts to drive a phase difference (i.e., frequency) betweentransmitter and receiver to zero. In this way, the output of NCO 165 maybe used to remove the residual frequency offset from the incoming signalto multiplier 145.

Note that some embodiments, a substantial amount of the components shownin FIG. 2 may be implemented in software. For example, in one embodimentLPF 140, CORDIC engine 150 and the feedback loop may be performed insoftware running on a DSP. To that end, embodiments may include anarticle in the form of a computer-readable medium onto whichinstructions are written. These instructions may enable the DSP or otherprogrammable processor to perform digital demodulation in accordancewith an embodiment of the present invention.

Using an embodiment such as that of FIG. 2, the demodulator output maythus be independent of the instantaneous frequency of the incomingsignal. In other implementations, a feedback loop of a receiver may beused to attempt to drive a phase value to zero. In other words, insteadof driving a phase difference (i.e., frequency) to zero, a phase of thevalue (i.e., the demodulating carrier phase relative to the incoming AMsignal) may be driven to zero. When this phase value is zero, thefrequency (i.e., phase difference) may also be zero. Thus embodimentsmay perform automatic frequency control (AFC) via a feedback loopincorporating a CORDIC function. In this way, a need for a dedicated AMdetector can be avoided.

Referring now to FIG. 3, shown is a block diagram of an AM receiver inaccordance with another embodiment of the present invention. As shown inFIG. 3, AM receiver 200 may include a front end having the same basicstructure described above with regard to FIG. 2. Specifically, as shownin FIG. 3, receiver 200 includes an antenna 205 to receive an RF signaland provide it to a low noise amplifier (LNA) 210. The output of LNA 110is provided to a complex mixer 220 which generates I and Q signalstherefrom. The complex outputs of mixer 220 may be amplified in PGAs 225a and 225 b. The output of PGAs 225 a and 225 b may be provided tocorresponding ADCs 230 a and 230 b. The outputs of ADCs 230 a and 230 bmay be provided to a DDFS 235 that may generate a downmixed basebandcomplex signal, which may in turn be filtered via a LPF 240. Thefiltered complex signal may then be multiplied in a combiner such as amultiplier 245 by a complex exponential. The output of multiplier 245may be provided to a CORDIC engine 250 to obtain both magnitude andphase information, i.e., polar coordinate information.

Note that the output of multiplier 245 is provided both to CORDIC engine250 and a splitter 255, which acts to pull the real portion of thecomplex value from the output of multiplier 245. This real portion thusoutput by splitter 255 may be the demodulated signal.

In the embodiment of FIG. 3, the output of CORDIC engine 250 (i.e., thephase output) may be provided to loop filter 260, which may act as a lowpass filter to filter the phase signal and provide it to NCO 265 thatgenerates the complex exponential that may then be provided tomultiplier 245 to remove the residual frequency offset to the input tomultiplier 245. In the embodiment of FIG. 3, improved demodulation mayoccur under poor signal conditions. For example, in cases of low signalto noise ratio (SNR) due to various conditions, improved performance mayresult using the embodiment of FIG. 3 over the embodiment of FIG. 2.

Using the embodiment of FIG. 2, the output of CORDIC engine 150, i.e.,the demodulated signal may still have an arbitrary phase. However, sinceonly the magnitude output from CORDIC engine 150 is used for thedemodulated signal, performance is not impacted. In the embodiment ofFIG. 3, in contrast, the output of CORDIC engine 250 may be phase lockedwith regard to the input signal. Note that while shown with theseimplementations in the embodiments of FIGS. 2 and 3, the scope of thepresent invention is not limited in this way. Furthermore, understandthat in many implementations, the CORDIC engine along with the feedbackloop (and low pass filter at the front end of the CORDIC engine) may beimplemented via hardware, firmware or software. Accordingly, in manyimplementations a DSP, general-purpose processor or other processingunit may perform the functions ascribed to these components viaexecution of software and/or firmware.

Note that because many of the components shown in both FIGS. 2 and 3 maybe implemented in software executing on generic hardware, a singlehardware implementation may include software that can execute inaccordance with both of the embodiments of FIGS. 2 and 3. That is,software may be present for both embodiments, and a controller mayselect an appropriate demodulation technique. For example, based ongiven signal conditions, the embodiment of FIG. 2 may be favored overFIG. 3, or vice versa. Of course, in other implementations a singlesoftware routine may be present to perform demodulation in accordancewith one of FIGS. 2 and 3. Still further, instead of software-basedoperation, actual hardware implementations in accordance with theembodiments of FIGS. 2 and 3 may be present instead.

In various embodiments, NCO 165 and 265 may be realized using the sameCORDIC functionality used for CORDIC engines 150 and 250. That is, insome implementations polar coordinate information may be input into theCORDIC algorithm, essentially operating backwards to provide rectangular(i.e., sine and cosine) outputs.

As mentioned above, in some embodiments an NCO may be implemented usingCORDIC functionality. Referring now to FIG. 4, shown is a block diagramof an NCO in accordance with an embodiment of the present invention. Asshown in FIG. 4, NCO 165, which may correspond to NCO 165 of FIG. 2, forexample, is coupled to receive an incoming signal, i.e., a voltagesignal N (e.g., output from loop filter 160 in the embodiment of FIG. 2)at a summer 166. Summer 166 is further coupled to receive a feedbackvalue from an output of a digital filter 167 that itself is coupled toan output of summer 166. Accordingly, the feedback loop may drive theoutput of summer 166 to generate a phase angle Φ out of filter 167. Inother embodiments, an accumulator may take the place of filter 167. Inturn, this phase value may be provided to a CORDIC engine 150, which maycorrespond to CORDIC engine 150 of FIG. 2. However, in the embodiment ofFIG. 4, CORDIC engine 150 may operate in reverse, receiving a polarcoordinate value, i.e., Φ, and generating rectangular coordinates,namely sine and cosine values therefrom. Specifically, the output ofCORDIC 150 may correspond to a complex exponential number, e.g., e^(jΦ).This value more specifically may be a negative exponential value whichmay be provided to a multiplier in order to remove frequency informationfrom an incoming signal also provided to the multiplier (e.g.,multiplier 145 of FIG. 2).

As shown in FIGS. 2 and 3, the demodulated output signal may beconverted back to an analog signal by a converter (not shown in FIGS. 2and 3) and provided directly out of a receiver and used as an audiosignal, e.g., to a speaker output, headphone output, recording deviceand so forth. However, additional signal processing may be performed onthe demodulated signal in some implementations. For example,particularly in cases where a receiver is mobile and moved during use,signal fading may occur. Thus, a given channel may suffer frominterference by various sources. As a result, during operation thevolume of a tuned channel may vary. While a user can increase ordecrease the volume of a device including the receiver (if volumecontrols are currently accessible to the user), this may be undesirableas the volume changes may occur rapidly, or may occur for only shortperiods of time. Or the volume changes may occur in relatively smallincrements so that a user does not notice the volume change for sometime. Accordingly, in some implementations automatic volume control(AVC) may be provided to further process a demodulated signal togenerate an audio signal with a substantially constant volume. Thisprocess may be performed automatically, i.e., transparently to a user,so that the user remains unaware of channel conditions causing a changein signal intensity (i.e., volume).

As described above, a received signal may have a value k, associatedwith the communication channel, which can vary with the environment. Inone embodiment, AVC may be implemented, in part by isolating k viatracking the kA term and adjusting it to keep it constant. Since AMtransmissions typically do not vary A, this method can provide suitableperformance. To obtain a statistic for kA, the other components of thetransmitted signal may be eliminated.

In embodiments using a CORDIC engine, the magnitude information obtainedby demodulation, i.e., k(A+m(t)), may also be used to perform automaticvolume control. Specifically, having obtained k(A+m(t)) a restrictionmay be imposed such that Em(t)=0. That is, the average energy of themessage information equals zero. Since m(t) is an audio waveform, it haslittle to no energy below 20 Hz. Thus a low pass filter with bandwidthof, e.g., 20 Hz or less may be used to recover kA and reject km(t).Changes in kA then may be tracked out to provide automatic volumecontrol.

Referring now to FIG. 5, shown is a block diagram of an AVC circuit inaccordance with one embodiment of the present invention. As shown inFIG. 5, AVC circuit 300 may receive an incoming demodulated signal. Asexamples, the received demodulated signal may come from one of receivers100 or 200, although the scope of the present invention is not limitedin this regard. For example, in other implementations a conventionalanalog AM receiver including a rectifier and RC filter may provide theAM demodulated signal to AVC circuit 300.

The demodulated signal is provided to an amplifier 310, which may be aPGA. PGA 310 may be controlled based on information from a feedbackloop. Specifically, as shown in FIG. 5, the output of amplifier 310 isprovided to a low pass filter (LPF) 320. LPF 320 may be used to filterout the message information, i.e., m(t), from the demodulated signal.Accordingly, the output of LPF 320 may correspond to channel-specificinformation. In other words, the output of LPF 320 may correspond to kA.Based on the value of kA, amplifier 310 may be controlled accordingly.

Specifically, a summer or comparator 335 may be coupled to receive theoutput of LPF 320. In addition, summer 335 receives a threshold level,which may correspond to a target volume. In various embodiments, thistarget volume may be set at a fixed amount, or may be adjusted based onvarious conditions, either by the receiver itself or under user control.The output of summer 335 is provided to a control input of PGA 310 tothus control the gain of PGA 310. Accordingly, if the signal is fading,kA becomes smaller and the gain provided by amplifier 310 may beincreased. In vice-versa operation, the gain of amplifier may beattenuated when signal strength is increasing in the channel.

The output of amplifier 310 is further coupled to a high pass filter(HPF) 330. HPF 330 may be used to remove the channel-specificinformation (i.e., kA) from the demodulated signal. Accordingly, theoutput of HPF 330 may correspond to the message information of thedemodulated signal, i.e., m(t). Further, because of the automatic volumecontrol provided by AVC 300, m(t) should remain at a steady level,regardless of input signal conditions, e.g., in cases of interferenceand so forth. Accordingly, using AVC circuit 300, embodiments mayprovide an output signal, e.g., to a speaker or other location, thatremains at a steady volume level even in the presence of a channel thatis impaired due to interference or another reason. While shown with thisparticular implementation in the embodiment of FIG. 5, the scope of thepresent invention is not limited in this regard. Furthermore, it is tobe understood that AVC circuit 300 may be implemented in hardware,software, firmware or combinations thereof. For example, a DSP thatperforms the demodulation of received signals, such as may be used toimplement the receivers of FIGS. 2 and 3, may further be programmed toperform AVC functionality. For example, PGA 310 (and the othercomponents) may be implemented in software. Thus PGA 310 may be effectedvia code that receives an incoming demodulated signal and multiplies itby a coefficient (e.g., from a lookup table) selected by the feedbackinformation (e.g., kA).

Referring to FIG. 6, in accordance with some embodiments of theinvention, the multimode transceiver 10 may be part of a multimediaportable wireless device 410, which, in turn, is part of a wirelesssystem 400. As examples, the wireless device 410 may be a dedicated MP3player, a cellular telephone or PDA with the capability of playing musicdownloads, part of a wireless link between a satellite antenna and an FMreceiver, etc.

Among its other various functions, the wireless device 410 may storedigital content on a storage 430, which may be a flash memory or harddisk drive, as a few examples. The wireless device 410 generallyincludes an application subsystem 460 that may, for example, receiveinput from a keypad 462 of the wireless device 410 and displayinformation on a display 470. Furthermore, the application subsystem 460may generally control the retrieval and storage of content from thestorage 430 and the communication of, e.g., audio with the multimodetransceiver 10. As shown, the multimode FM transceiver 10 may bedirectly connected to speakers 440 and 450 for output of audio data. Asdepicted in FIG. 6, the multimode FM transceiver 10 may be coupled by amatching network 434 to a receiver antenna 480 and may be coupled by amatching network 432 to the transmit antenna 482.

Although the wireless device 410 may include the speakers 440 and 450,it may be desirable to play sounds that are generated by the wirelessdevice 410 over a more sophisticated speaker system. Therefore, inaccordance with some embodiments of the invention, the wireless device410, via the multimode FM transceiver 10, may broadcast content to beplayed over an FM channel to the receiver of an adjacent stereo system500 (as an example). As shown, the stereo system 500 includes an RFantenna 504 for purposes of receiving the transmitted content from thewireless device 410.

In accordance with some embodiments of the invention, the wirelessdevice 410 may have the ability to communicate over a communicationsnetwork, such as a cellular network. For these embodiments, the wirelessdevice 410 may include a baseband subsystem 475 that is coupled to theapplication subsystem 460 for purposes of encoding and decoding basebandsignals for this wireless network. Baseband subsystem 470 may be coupledto a transceiver 476 that is connected to corresponding transmit andreceive antennas 477 and 478.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: receiving an amplitude modulation (AM) signal ina receiver; performing a coordinate rotation digital computer (CORDIC)operation in obtaining a demodulated AM signal; and providing a phaseoutput of the CORDIC operation to a feedback loop to drive a phasedifference between the AM signal and a demodulating carrier signal ofthe feedback loop to zero.
 2. The method of claim 1, further comprisingproviding a magnitude output of the CORDIC operation as the demodulatedAM signal.
 3. The method of claim 1, further comprising providing thephase output to the feedback loop to drive a phase of the demodulatingcarrier signal relative to the AM signal to zero.
 4. The method of claim1, further comprising converting the received AM signal to a complexbaseband signal and multiplying the complex baseband signal by thedemodulating carrier signal of the feedback loop.
 5. The method of claim4, further comprising providing a magnitude output of the multiplying asthe demodulated AM signal.
 6. The method of claim 1, further comprisingsumming the phase output with a feedback signal to obtain a summed phasevalue, filtering the summed phase value and performing a second CORDICoperation on the filtered summed phase value to generate thedemodulating carrier signal of the feedback loop, the demodulatingcarrier signal corresponding to a complex exponential signal.
 7. Anapparatus comprising: a receiver to receive an amplitude modulation (AM)signal and generate a complex baseband signal therefrom; a combiner toreceive the complex baseband signal and a demodulating carrier signal ofa feedback loop and to generate a complex output therefrom; and acoordinate rotation digital computer (CORDIC) engine to perform a CORDICfunction on the complex output to obtain a polar result.
 8. Theapparatus of claim 7, wherein a magnitude of the polar result comprisesa demodulated signal.
 9. The apparatus of claim 7, further comprising acontroller to select an output of the combiner or an output of theCORDIC engine to provide a demodulated signal.
 10. The apparatus ofclaim 7, wherein the CORDIC engine comprises a software routineperformed by a processor.
 11. The apparatus of claim 7, wherein thefeedback loop is coupled to an output of the CORDIC engine to generatethe demodulating carrier signal comprising a complex exponential signalbased on a phase of the polar result.
 12. The apparatus of claim 11,wherein the feedback loop comprises: a loop filter coupled to an outputof the CORDIC engine; and a controlled oscillator coupled to an outputof the loop filter, wherein the controlled oscillator is to generate thecomplex exponential signal, wherein the complex exponential signal is tobe coupled to an input of the combiner.
 13. The apparatus of claim 12,wherein the feedback loop further comprises a differentiator coupledbetween the CORDIC engine and the loop filter.
 14. The apparatus ofclaim 12, wherein the controlled oscillator comprises a numericallycontrolled oscillator including: a summer to receive and sum the outputof the loop filter and a feedback signal; a filter to receive and filterthe summed output to generate a filtered output, wherein the feedbacksignal corresponds to the filtered output; and the CORDIC engine toreceive the filtered output and generate a rectangular result.
 15. Theapparatus of claim 12, wherein the feedback loop is to enable automaticfrequency control of the AM signal without a dedicated detector.
 16. Theapparatus of claim 7, wherein the apparatus comprises a multimodereceiver including an analog front end to receive and downconvert the AMsignal to an intermediate frequency (IF) digitized signal and aprocessor to downconvert the IF digitized signal to baseband and toimplement the CORDIC engine.
 17. The apparatus of claim 16, wherein themultimode receiver is operable in an AM mode or a frequency modulation(FM) mode, based on user control.
 18. A system comprising: a digitaldemodulator to perform digital demodulation on an amplitude modulation(AM) signal, the digital demodulator to receive a complex basebandsignal generated from the AM signal and perform a coordinate rotationdigital computer (CORDIC) operation on the complex baseband signal togenerate polar information; a feedback loop coupled to receive a phaseportion of the polar information and generate a complex exponentialsignal for input into the digital demodulator; and an output device tooutput an audio signal generated from the digital demodulation.
 19. Thesystem of claim 18, further comprising a converter to convert amagnitude portion of the polar information to obtain the audio signal,wherein the magnitude portion comprises a demodulated signal.
 20. Thesystem of claim 18, wherein the digital demodulator comprises: acombiner to combine the complex baseband signal and the complexexponential signal; and a CORDIC processor coupled to an output of thecombiner to perform the CORDIC operation.
 21. The system of claim 20,wherein the feedback loop comprises: a loop filter coupled to an outputof the CORDIC processor; and a controlled oscillator coupled to anoutput of the loop filter, wherein the controlled oscillator is togenerate the complex exponential signal.
 22. The system of claim 21,wherein the controlled oscillator comprises a numerically controlledoscillator including: a summer to receive and sum the output of the loopfilter and a feedback signal; a filter to receive and filter the summedoutput to generate a filtered output, wherein the feedback signalcorresponds to the filtered output; and the CORDIC processor to receivethe filtered output and generate a rectangular result.
 23. The system ofclaim 20, wherein the feedback loop is to enable automatic frequencycontrol of the AM signal without a dedicated detector.